1. Field of the Invention
The present invention relates to the field of the crystal growth of epitaxial films and substrates. In particular, the invention relates to the crystal growth of thermally mismatched epitaxy and substrate.
2. Description of Prior Art
Although heteroepitaxial crystal growth has been used for several decades, it is only recently that epitaxial growth in large mismatch systems has been examined. Examples of some mismatched material systems include GaAs epitaxy on Si substrates, and GaN epitaxy on sapphire substrates. The major concern in these systems has been the lattice mismatch between the epitaxy and substrate. The lattice mismatch is the difference in the appropriate fundamental crystallographic unit length (called the lattice constant) of the substrate and epitaxy. If this mismatch is too large the substrate does not form an acceptable template for the desired epitaxy. A poor substrate template results in epitaxy that contains a large defect density or epitaxy that is not oriented with respect to the substrate and is most often multi-domained. If the lattice mismatch is small the epitaxy will be commensurate, but the strain energy will increase with epitaxial thickness until the epitaxy relaxes through dislocation generation.
However, in some cases a large thermal expansion mismatch between the substrate and epitaxy is present and can be just as problematic as the lattice mismatch described above. The thermal expansion mismatch between the epitaxy and substrate is the difference in the change in lattice constants of the two materials with temperature. One technique for avoiding thermal mismatch problems is to remove the substrate entirely at the growth temperature. For example, U.S. Pat. No. 5,679,152 to Tischler et al. discloses a method of making a single crystal GaN substrate by epitaxially depositing the GaN on a growth substrate. At the growth temperature the substrate is completely etched away either before or after the GaN deposition is complete. When the GaN layer is then cooled, there is no thermal mismatch because the substrate is no longer present. This technique, however, has several disadvantages. Because the technique requires that the substrate is completely removed, significant etching is required by this approach. This etching adds cost and complexity to the process. In addition, it can also be difficult to hold the GaN layer in place after the sacrificial substrate is etched away.
It is instructive to note that other substrate removal processes have been used for various purposes unrelated to the thermal mismatch problem. These include the removal of the substrate from the epitaxy by etching or polishing away the substrate after the substrate-epitaxy system has been cooled. Thermal mismatch problems are not solved by such techniques. The motivation for this post-growth removal process is either to reduce optical absorption by the substrate or reduce resistive heating in the epitaxial layer by providing a better heat sink directly to the epitaxial layer. These two processes are motivated by electronic or optoelectronic device performance and are not related to structural issues of the epitaxy.